I have a Shih-tzu that started the drinking to much water and obsessed with food. After many costly tests etc they concluded that he has "Cushings Disease". Had him on a very expensive medicine for a few months and it only helped some so I stopped them. Now he is better but never will be "normal". We will just love him as much as we can for as long as we can. He isn't in any pain or real discomfort. Then he had what we thought was a stroke but the vet called it "Old Dog Vestubular" (google it). Which says he has some kind of tumor in the inner ear or brain. His little head tilts to the side now but he is doing pretty good. My point is it may not be the food at all so I would recommend taking her to a vet. Get your pocketbook out though as they are not cheap. But if you can extend her life and she is happy it is worth it.